Bacterial pathogens induce a broad set of virulence loci upon contact with their hosts. In response to this attack, a host will induce its own set of complex defenses. In this arms race, defensive molecules produced by one organism may lead to the evolution of counter-defensive molecules by the other. An elucidation of the mechanism of virulence requires the comprehensive identification any analysis of the function and evolution of virulence loci. An in vivo expression assay will be used to identify virulence loci that are induced in the pathogen during all stages of the infection process in the model system of the phytopathogen Pseudomonas syringae, and the plant Arabidopsis thaliana. The function of virulence determinants will be characterized by sequence analysis, database searches and knockout experiments. Genes that are important during the initial pathogen attack early in infection will be differentiated from genes whose products may function as counter-defenses to the hostile plant defensive environment. In the long term an evolutionary genetic analysis will be applied to the set of virulence determinants identified in this study in order to gain a complete understanding of the mechanisms and origins of virulence in Pseudomonas pathogens of plants and animals.